jf-common: Further loki.sh refinements

Based on discussions in 43376, add a check for unlocked bootloaders
and flash a virgin boot.img on those devices.  Since no unlocked
bootloaders exist I am aware of, the unlocked_bootloaders is
currently empty.  From egrep man page:

The empty file contains zero patterns, and therefore matches nothing.

This should function the same as the current version then.  And will
only flash virgin boot images as unlocked_bootloaders is updated
to include them.

Change-Id: Ia846e4f9f361eb89cc0aa38cb5a7b0d93781b73f